The present invention relates to a method for purifying polysaccharides present in the form of an aqueous solution.
There have been known methods for purifying polymers present in solutions as solutes, which comprise precipitating the polymers through addition of other solvents which are miscible with the solvents for the polymer solutions, but do not solubilize the polymers to give purified polymers.
The foregoing method has been tried to apply to the purification of xanthane gum which is a polysaccharide obtained through fermentation by microorganism. According to this method, the xanthane gum is precipitated and separated by admixing the culture medium containing xanthane gum with isopropanol as a hydrophilic solvent. This method, however, requires the use of a large amount of isopropanol. Therefore, it is very troublesome to recover the isopropanol after the precipitation and separation of the xanthane gum and too much expenses must be spent upon the equipment and facilities for the treatment. Moreover, the xanthane gum is precipitated from the solution in the form of strings and if a stirring machine is used, it is difficult to perform the purification operation since the xanthane gum coils round the stirring machine.
There have been proposed various techniques for solving these problems. For instance, Japanese Examined Patent Publication (hereinafter referred to as "J. P. KOKOKU") No. Sho 54-14679 discloses a method which comprises the step of adding a dilute aqueous solution of isopropanol to a culture medium containing xanthane gum; and J. P. KOKOKU No. Hei 1-4521 discloses a method comprises adding isopropanol in portions. These methods can provide xanthane gum as precipitates which do not have a string-like shape, but require the use of a large quantity of isopropanol and this in turn makes the recovery of the isopropanol difficult.
Further, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Hei 2-255801 discloses a method for purifying polymers present in solutions, which comprises precipitating the polymers in a mixing and stirring apparatus provided with two (i.e., upper and lower) stirring blades and simultaneously pulverizing the precipitated polymers into small masses. According to this method, however, the ability of the stirring machine to pulverize the precipitated polymers into small masses is greatly dependent upon the residence time of the polymers in the apparatus and correspondingly the processing rate is greatly limited.